This invention relates to image intensifier devices and more particularly to a method of manufacturing gallium arsenide photocathodes.
Image intensifier devices multiply the amount of incident light they receive and thus provide an increase in light output which can be supplied to a camera or directly to the eyes of a viewer. These devices are particularly useful for providing images from dark regions and have both industrial and military application.
The image intensifier device utilizes a photoemissive wafer which is bonded to a glass faceplate to form a photocathode. Light enters the faceplate and strikes the wafer, thereby causing a primary emission of electrons. The electrons are accelerated across a gap by an electric field to a microchannel plate. The microchannel plate amplifies the initial electron current from the cathode by approximately 300 times. A phosphor layer deposited on an output window functions as an anode.
At the bonding stage, the photoemissive wafer has three major layers, namely, a silicon nitride (Si.sub.3 N.sub.4) antireflection layer, a gallium aluminum arsenide "window" layer and a gallium arsenide active layer. A silicon dioxide (SiO.sub.2) layer deposited on the silicon nitride layer provides an interface between the wafer and the faceplate during bonding.
After the bonding step, a metallic conductive layer is applied to a portion of the outer surface of the photocathode. The photocathode is then heat cleaned to remove contaminants, such as oxygen and carbon, from the gallium arsenide layer.
A major step in the processing of the photocathode is an activation step. The activation step includes the deposition of cesium oxide on the outer gallium arsenide layer of the wafer, thereby reducing the work function and permitting the electrons to be more easily released from the gallium arsenide layer.
The cathodes are placed inside a vacuum system during the deposition stage. Electrical contact is made to the metallic conductive layer of the cathode to monitor the photo response of the cathode during deposition. If no photoemission is detected, the process is aborted and the photocathode scrapped resulting in considerable expenditure of time and money.
Failure of the deposition process may result from one or more reasons. Three major reasons are (1) the cathode is contaminated, (2) cesium is not being released into the vacuum system and (3) electrical contact is not being made to the conductive layer. Elimination of any one of the reasons would result in a considerable saving in manufacturing cost and effort. Up to now it has been nearly impossible to determine the specific reason or reasons for deposition failure, thereby resulting in the discarding of cathodes which could otherwise have been used.
It is therefore an object of the present invention to provide an apparatus for eliminating one cause of photo response failure during activation of a photocathode.
It is an additional object of the present invention to provide a method of performing an activation step which provides increased production of photocathodes and savings in manufacturing costs.